Keep it at the cheap side. Invest the extra money in a flash and a tripod (my personal order of priorities); both are accessories that are not cheap (usually, you get what you pay for).
Personally I consider the WR a second line of defense; common sense and, if needed, a lens coat or plastic bag will also do. With that in mind, I would consider the Sigma 17-70 Contemporary (currently $400) as well. Actually, it would be what I would buy if I would start out today with a Pentax camera and a standard zoom.
From the review here (
Sigma 17-70mm F2.8-4 Contemporary Review - Conclusion | PentaxForums.com Reviews Quote: We'll get straight to the point and say that if we were limited to having only one walkaround lens, this would be the one. While it does not offer quite as much reach as the Pentax 18-135mm, we prefer it for is versatility (faster aperture and macro capabilities) and improved image quality. If only it were weather-sealed, it would be the perfect lens! If this lens is indicative of the quality and performance of other lenses in Sigma's new lineup, then the Pentax community will surely be in for a treat.